Rockwool
An inert substrate spun from molten basalt into fibers and pressed into cubes and slabs. It holds a large volume of water and is used almost entirely in drain-to-waste systems.
Rockwool is made by melting basalt and spinning the melt into fine fibers, which are then bound into blocks and slabs. The result is chemically inert and holds no nutrition of its own, so everything the plant receives comes from the feed.
Its water release behavior is what makes it useful for steering. A slab holds a large volume of water and gives up the first portion readily, then holds the remainder more tightly, which makes dryback percentages a precise and repeatable lever compared with soil.
Raw rockwool is alkaline and has to be conditioned to a target pH before transplant, or the first weeks of feed go into correcting the substrate rather than feeding the plant.
Because a rockwool program is built on measured dryback curves, anything that changes transpiration changes the curve underneath it. CO2 enrichment slows dryback, so a program calibrated at ambient has to be re-baselined once the gas is running.